The present invention relates to ballast circuits for starting gas discharge lamps, and, more particularly, to an improved ballast circuit of the reactor type.
Conventional fluorescent lamp ballasting circuits typically include an autotransformer to produce a voltage of sufficient amplitude between the cathodes of a fluorescent lamp to produce an arc therebetween in order to start the lamp and cause it to remain illuminated. The primary winding of an autotransformer, however, has a coil resistance that results in a loss of electrical energy. Therefore, if the autotransformer can be eliminated from the ballast circuit, the efficiency of the ballast would be greatly increased. Furthermore, the autotransformer represents a significant expense item in the production of conventional ballast circuits.
A reactor technology-type of ballast circuit having the line voltage directly connected to an inductive ballasting coil without an intermediate autotransformer may be used to start and sustain illumination of a fluorescent lamp if the lamp is capable of operating at voltages slightly below the line voltage (i.e. the lamp is rated to operate at a voltage within about 65% of the expected line voltage). Because of the higher line voltages involved, reactor technology ballasts of this type are generally only recommended for use with biaxial-shaped and circular-shaped lamps which have their end cathodes proximate to each other, such as the Biax.TM. and Mod-U-Line.TM. family of lamps as manufactured by the General Electric Company, Cleveland, Ohio.
Conventional ballast circuits also usually provide an arrangement for supplying pre-heating current to the lamp cathodes. Pre-heating the lamp cathodes prevents "sputtering" which is erosion of the cathode metal each time an arc is generated to start the lamp when the cathodes are not pre-heated to an adequate thermionic temperature. A method of providing cathode pre-heat current is to electrically connect a glow type thermal switch between the lamp cathodes. The glow switch produces a glow discharge to heat the switch's thermally responsive contacts which then close to conduct heating current to the lapp cathodes when line voltage is initially applied to the ballast circuit as described in U.S. Pat. No. 2,740,861 to Lake. The switch glow discharge path is shunted when the switch contacts make, and heat is no longer generated to hold the contacts closed. After a time delay, the contacts should cool and open to start the lamp, but on occasion have been known to remain closed ballast to overheat and become a potential overheating hazard.
Conventional ballast circuits also usually have a power factor correction capacitor in series with one of the inductive ballasting coils to reduce the energy consumption of the ballast circuit during operation of the lamp. Prior to lamp starting, however, the capacitor can prevent the lamp in an electrical path with it from receiving a sufficient level of cathode pre-heat current. Thus, the lamp can be started before the lamp's cathodes have reached an adequate thermionic temperature and sputtering can result, thereby shorten the lamp's serviceable life.
It is accordingly an object of the present invention to provide an improved ballast circuit which is not subject to the foregoing disadvantages.
It is another object of the present invention to provide an improved, more energy efficient reactor-type ballast circuit for a fluorescent lamp.
Another objective is to provide a ballast circuit of the above character, which is simple in construction, inexpensive to manufacture, and reliable over a long service life.
Other objects of the invention will in part be obvious and in part appear hereinafter.